The southwestern United States has substantially changed since 1900, with warming temperatures, diminishing precipitation and streamflows, worsening drought, and growth in population, irrigated agricultural land, and water withdrawals (MacDonald, 2010). The region is currently experiencing its worst drought in recorded history, with the period from 2000 to 2018 being the driest 19-year period since the late 1500s and the second driest since 800 Common Era (Williams et al., 2020). Despite little change in precipitation in the Upper Colorado River Basin (UCRB) between 1896 and 2019, temperatures have risen (Tillman et al., 2020) and water supplies in the basin have suffered (Udall & Overpeck, 2017). This recent drought has strained water resources and led to Congress passing the Colorado River Basin Drought Contingency Plan (DCP) Authorization Act in 2019. The DCP outlines agreements for Colorado River Basin (CRB) stakeholders to collaborate to reduce the risk that large Colorado River reservoirs (Lake Powell and Lake Mead) decline to critically low elevations. Despite these efforts, both reservoirs reached historic low elevations in 2021 (Bureau of Reclamation, 2021a, 2021b).
The Colorado River is a critical water resource in the southwestern United States, supplying drinking water for 40 million people in the region and water for irrigation of 2.2 million hectares of land. Extended drought in the Upper Colorado River Basin (UCOL) and the prospect of a warmer climate in the future pose water availability challenges for those charged with managing the river. Limited water availability in the future also may negatively affect aquatic ecosystems and wildlife that depend upon them. Water availability components of special importance in the UCOL include streamflow, salinity in groundwater and surface water, groundwater levels and storage, and the role of snow in the UCOL water cycle. This manuscript provides a review of current “state of the science” for these UCOL water availability components with a focus on identifying gaps in data, modeling, and trends in the basin. Trends provide context for evaluations of current conditions and motivation for further investigation and modeling, models allow for investigation of processes and projections of future water availability, and data support both efforts. Information summarized in this manuscript will be valuable in planning integrated assessments of water availability in the UCOL.
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